The present invention relates to a thermal line printer of the type having a segmented line head composed of a plurality of linear thermal head segments aligned in the widthwise direction of a recording medium sheet perpendicular to the feeding direction or the lengthwise direction of the recording medium sheet so as to cover the entire span or width of the recording medium sheet.
One type of conventional segmented line head is composed of a plurality of linear thermal head segments aligned in a single row and coupled to each other at opposed ends of adjacent segments. Each linear segment has a given length sufficient to cover the span of A4 or B4 size recording paper and is formed with a plurality of heating elements arranged linearly on the segment at a given pitch. These linear head segments are connected in series to each other to constitute the line head which can cover the entire length of larger size recording paper such as A1 size and A0 size, and which has a higher yield rate than that of monolithic line head of comparative length.
However this type of conventional segmented line head has the drawback that the pitch of the heating elements is made irregular at the junction or connecting portion of adjacent segments which degrades the quality of the printed image pattern.
Another type of conventional segmented line head is disclosed in U.S. Pat. No. 4,660,052. This conventional head is composed of a plurality of linear thermal head segments aligned in a pair of parallel rows in staggered relation and in partially overlapping relation at end portions of the linear segments between the parallel rows so as to completely cover the entire width of recording paper. In operation, the first row of linear segments is activated to effect a part of single line printing, and then the recording paper is advanced through an interval corresponding to the distance between the parallel rows and the second row of linear segments is activated to effect the remaining part of the single line printing to thereby complete the single line printing. In such operation, in order to avoid duplicate printing by the overlapping portion of the staggered segments between the first and second rows, a predetermined number of heating elements are blanked during each line printing operation at the overlapping portion of the staggered linear segments.
During the starting period of the line-sequential operation in the vertical direction, a head segment in the downstream row is blanked until the head segment reaches the starting line printed by another head segment in the upstream row through the vertical distance or gap between the pair of rows. The line thermal printer is provided with a line memory for storing bit image data to be distributed to the head segments. A buffer line memory is also provided for temporarily storing bit image data to be distributed to the head segment in the downstream row during the starting period. The capacity of the extra buffer memory is determined according to the gap between the pair of parallel rows.
FIG. 3 shows a staggered arrangement of the head segments in the prior art. An individual head segment 11 is comprised of a longitudinal substrate formed with a plurality of electrically resistive heating elements aligned linearly along a lengthwise center line l1a substrate. Such structure of the head segment is referred to as "center type" hereinafter. As shown in FIG. 3, the first or upstream row of the linearly aligned hearing elements is spaced from the second or downstream row of the linearly aligned heating elements a relatively long distance in the vertical direction when utilizing the center type segments. The more the distance between the parallel rows of the heating elements, the more the extra memory area or capacity needed to store temporarily the image bit data. Generally, the line memory of the type used in the thermal line printer is very expensive in view of its great memory capacity to meet the width of the recording medium and the line density in the lengthwise direction thereof. Further, the more the gap between the parallel rows of the heating elements, the more the complicated control structure is needed to align linearly the dots printed by the parallel rows of heating elements.